The efficiency of processes which use light to make physical and/or chemical changes in material may depend upon the amount and/or the spatial distribution of the light, which in turn can be affected by the sizes, configurations and positioning of light sources, reflectors and objects to be treated. The effect of changes in equipment, such as structural changes in light sources and reflectors, can be determined by appropriate instruments. However, there is a need for instruments to monitor light which is highly concentrated in a small zone and/or which converges on an object to be treated through a 360.degree. arc, such as light which originates at one focus of an elliptical reflector and is directed towards an object at the other focus.